Automobiles, airplanes and other vehicles commonly feature disk brakes because of their advantages over drum brakes. Disk brake pads apply a flat, relatively small contact surface to a rotor as compared to a curved, larger brake shoe surface, so that disk brakes allow more precise control of braking action. Brake pad footprint on the rotor compared to total rotor surface is small, compared to the proportion of a brake drum in contact with shoes, so that disk brakes cool more efficiently and quickly and accordingly fade less than drum brakes.
Recent developments in automobile front wheel drive suspensions require thinner brake rotors and thus closer parallelism and runout tolerances. Increased use of metallic components in brake pads additionally requires far closer tolerances and a more perfect rotor finish to prevent squealing and other detrimental affects of harmonics.
Recent developments in anti-lock braking systems impose additional requirements for precision tolerances in brake rotors. Such systems employ sensors that gauge the pad-rotor gap to provide a feedback signal. That signal automatically adjusts brake pressure in order to reduce skidding and increase braking control. Scoring, runout and other imperfections in rotor shape and finish thus present erroneous signals and feedback to the detriment of braking control. Many automobile manufacturers accordingly require that brake rotors have a "non-directional" finish so that the rotor faces present as closely as possible a perfect planar surface to the brake pads.
Earlier brake rotor finishing devices employ several techniques in an effort to impart a "non-directional" finish to brake rotors. Although the adjective "non-directional" is commonly used in the automobile industry for rotor finishes which present no concentric, spiral or collinear scoring, typical "non-directional" finishers actually abrade rotors in many directions at a particular instant, but ideally accentuate finishing in no particular direction. This document refers to such finishers and finishes as "non-directional," consistent with industry custom.
One previous non-directional finisher applies a finish while the rotor remains in place on the automobile brake assembly. The device includes a frame which attaches to the brake caliper mount. The frame suspends and positions two coaxial, parallel abrading disks against the rotor surfaces. An adjustment knob allows control of pressure of disks against rotor. The disks place a non-directional finish on both faces of the rotor as they spin from friction imparted by the rotating rotor. This type of finisher, however, constrains the disks from translating (or moving laterally) in any direction and from rotating in any direction except to spin. Any imprecision in alignment between the caliper mount (and thus the finisher) and the rotor axis thus introduces errors and imperfections into the rotor finish. Such misalignment frequently exaggerates grinding action in particular directions on the rotor face, for example, and thus causes an imperfect non-directional finish as well as introducing error in planar flatness.
A second previous rotor finisher includes a mounting bracket that attaches to a brake lathe. The bracket supports an axle that carries a single abrasive disk. The bracket and axle force the disk against the rotor, and rotation of the rotor on the lathe causes the disk to rotate. The bracket constrains the axle (and the disk) from translating in any direction, so that misalignment of the bracket on the lathe introduces errors into the rotor finish.
A third previous rotor finisher includes a rotatable abrasive disk mounted perpendicularly to a handle at the end of a drive cable. The user connects the drive cable to a power source and manually applies the disk to the brake drum. This device obviously introduces the possibility of random errors in finish direction, runout, parallelism and flatness.
Other devices which have been used to finish brake rotors include sandpaper coated wood blocks and small abrasion pads attached to tong-like devices. Such devices can cause concentric scoring as well as errors in runout and parallelism.